Hybrid imaging technologies of nuclear medicine imaging and computer tomography such as Positron Emission Tomography/X-ray Computed Tomography (PET/CT) and more recently Single Photon Emission Computed Tomography/CT (SPECT/CT) are well-known. PET/Medical Resonance Imaging (PET/MRI), however, offers several important advantages over PET/CT. Unlike PET/CT where imaging is sequential, PET/MRI enables true simultaneous multi-modality measurement (although there are PET/MRI solutions utilizing field-cycled MRI offering temporally-interleaved PET and MRI). Compared with CT imaging, the technology of magnetic resonance imaging offers much larger breadth in measurement parameters. The well-established measurements of longitudinal and transverse magnetization relaxation (R1, R2) and proton density offer superior soft tissue contrast compared with CT.
Although MRI has difficulties imaging bones due to lack of water content, the marrow can be seen and the bones easily inferred. In MRI, pulse sequences allow for functional imaging (fMRI). In the brain the functional imaging encompasses the detection of blood oxygenation level dependent (BOLD) tissue changes as described in “Intrinsic signal changes accompanying sensory stimulation: functional brain mapping with magnetic resonance imaging” authored by Ogawa et al. (Proc Natl Acad Sci USA 89 5951-5, 1991) and quantitative measurements of blood flow using arterial spin labeling (ASL) as described in “Effects of the apparent transverse relaxation time on cerebral blood flow measurements obtained by arterial spin labeling” authored by St. Lawrence et al. (Magn Reson Med 53 425-33, 2005). In the heart, the functional imaging encompasses contrast agent bolus tracking and wall motion studies as described in “Myocardial blood flow quantification with MRI by model-independent deconvolution” authored by Jerorsch-Herold M et al. (Med Phys 29 886-897, 2002). Further, MRI can measure water diffusion amplitude (“coefficients”) as described in “Current status of body MR imaging: fast MR imaging and diffusion-weighted imaging” authored by Koyama et al. (Int J Clin Oncol 11 278-285, 2006), tensors enabling tractographic imaging as described in “In vivo Fiber Tractography using DT-MRI Data” authored by Basser et al. (Mag Reson Med 44 625-632, 2000), and spectroscopy to measure biochemistry. Another advantage of MRI over CT concerns the ionizing radiation association with the latter (typical dose from helical CT of the chest is about 8 mSv, resulting in a 0.03% increased risk of stochastic induction of fatal cancer). Although a typical PET study results in about a 14 mSv dose, the lack of MRI ionizing radiation is of value in serial and pediatric studies. This is not to say, however, that hybridization of PET with MRI is without challenges.
To date, efforts to hybridize modalities has resulted in popular clinical PET/CT and SPECT/CT platforms. There is substantial overlap between SPECT and PET clinical capabilities, with each modality having advantages and disadvantages. SPECT has the advantage of being able to image multiple isotopes simultaneously. Single photon emitting radioisotopes generally have longer radioactive half-lives than their PET counterparts which has implications for radiopharmaceuticals with slow binding kinetics as described in “A reappraisal of the relative merits of SPET and PET in the quantification of neuroreceptors: the advantages of a longer half-life!” authored by Lassen (Eur J Nucl Med 23 1-4, 1996). On the other hand, clinical PET images have superior resolution and sensitivity compared with clinical SPECT, although research-grade pre-clinical SPECT systems (small field-of-view systems for imaging small animals) can outperform pre-clinical PET systems due to positron range issues as described in “Instrumentation for Molecular Imaging in Preclinical Research: Micro-PET and Micro-SPECT” authored by Chatziioannou (Proc Am Thoracic Soc 2 533-536, 2005).
PET, like SPECT, suffers from the physical phenomena of gamma-ray attenuation and scattering. These emission tomography modalities rely on detection of gamma-rays emitted via decay of a radioisotope (bound to a radiopharmaceutical) that is injected into the subject to be imaged. From their point of isotropic emission, the gamma-rays travel through the subject interacting with the tissues of the subject (specifically, the electrons) along the way. During travel, some gamma-rays are absorbed by tissue or are scattered away from the detectors and hence are lost or “attenuated”, or are spatially mis-positioned (“scattered”). The obstacles of attenuation and scatter have received much attention over the past decades. Arguably, these obstacles have been overcome for specific circumstances in SPECT and PET, with increasingly sophisticated techniques offering diminishing returns. Attenuation compromises the quantitative accuracy of PET, and correcting for attenuation is a major area of research. In principal, the effects of attenuation can be corrected if the distribution of attenuation coefficients is known. In the past, this was accomplished by hybridizing PET with X-ray CT, yielding the PET/CT apparatus. This hybridization was convenient because CT measurements are easily related to electron density and hence the distribution of attenuation coefficients could be provided with relative ease. In Hybrid MRI/PET, however, there is significant challenge to deriving the distribution of attenuation coefficients, since MRI measurements using contemporary clinical MRI are not easily related to electron density.
The unification of different imaging modalities into a single Hybrid platform is disclosed in “Myocardial Perfusion Imaging with a Combined X-ray CT and SPECT System” authored by Kalki et al. (J Nucl Med 38 1535-1540, 1997) for SPECT/CT and for Positron Emission Tomography (PET) PET/CT in “A Combined PET/CT Scanner for Clinical Oncology” authored by Beyer et al. (J Nucl Med 41 1369-1379, 2000). In both cases, X-ray CT data guided attenuation correction of the gamma-ray measurements.
U.S. Pat. No. 5,672,877 to Liebig et al. discloses a method of coregistering medical image data of different modalities. In the method, an emission scan of an object is performed using a nuclear medicine imaging system to acquire single-photon emission computed tomography (SPECT) image data. A transmission scan of the object is performed simultaneously with the emission scan using the same nuclear medicine imaging system in order to acquire nuclear medicine transmission image data. The emission scan is performed using a roving zoom window, while the transmission scan is performed using the full field of view of the detectors. By knowing the position of the zoom windows for each detection angle, the nuclear medicine transmission image data can be coregistered with the SPECT emission image data as a result of the simultaneous scans. Image data of a modality other than SPECT, such as x-ray computed tomography (x-ray CT) data, magnetic resonance imaging (MRI) data, or positron emission tomography (PET) data, is also provided, which it is desired to have coregistered with the SPECT emission data. The nuclear medicine transmission image data is therefore coregistered with the image data of the different modality. As a result, the image data of the different modality becomes coregistered with the SPECT image data.
U.S. Pat. No. 7,286,867 to Schlyer et al. discloses a combined PET/MRI scanner including a magnet for producing a magnetic field suitable for magnetic resonance imaging, a radiofrequency (RF) coil disposed within the magnetic field produced by the magnet and a ring tomograph disposed within the magnetic field produced by the magnet. The ring tomograph includes a scintillator layer for outputting at least one photon in response to an annihilation event, a detection array coupled to the scintillator layer for detecting the at least one photon outputted by the scintillator layer and for outputting a detection signal in response to the detected photon and a front-end electronic array coupled to the detection array for receiving the detection signal. The front-end array has a preamplifier and a shaper network for conditioning the detection signal.
U.S. Pat. No. 6,927,406 to Zyromski discloses a multimodal source for imaging with at least one of a gamma camera, a positron emission tomography (PET) scanner and a single-photon-emission computed tomography (SPECT) scanner, and at least one of a computed tomography (CT) scanner, magnetic resonance imaging (MRI) scanner and optical scanner. The multimodal source has radioactive material permanently incorporated into a matrix of material, at least one of a material that is a target for CT, MRI and optical scanning, and a container which holds the radioactive material and the CT, MRI and/or optical target material. The source can be formed into a variety of different shapes such as points, cylinders, rings, squares, sheets and anthropomorphic shapes. The material that is a target for gamma cameras, PET scanners and SPECT scanners and/or CT, MRI and/or optical scanners can be formed into shapes that mimic biological structures.
U.S. Pat. No. 7,378,660 to Case et al. discloses a computer program, method, and system to facilitate hybrid CT attenuation correction. In one embodiment, the method generally includes acquiring data from a scanner, utilizing an ordered subset expectation maximization-bayesian algorithm to reconstruct the acquired data, and forward projecting the reconstructed data. Such a configuration minimizes the computing resources required for reconstruction and improves attenuation correction accuracy.
U.S. Pat. No. 7,348,564 to Wollenweber et al. discloses a method for correcting emission data from Positron Emission Tomography (PET) or SPECT and includes generating a plurality of computed tomography (CT) image data, selecting a portion of the CT image data, processing the selected CT data to generate a plurality of attenuation correction (CTAC) factors at the appropriate energy of the emission data, weighting the CTAC factors to generate an emission attenuation correction map, wherein the weights are determined based on the axial location and the slice thickness of the CT data and the axial location and the slice thickness of the PET or SPECT data, and utilizing the generated attenuation correction map to generate an attenuation corrected PET image.
U.S. Pat. No. 6,628,983 to Gagnon discloses a nuclear imaging system comprising a transmission radiation source that radiates at a plurality of energy levels within a specified energy range. The energy range is divided into two or more energy subranges. Detectors detect the position or trajectory and energy of transmitted radiation and emitted radiation. A sorter sorts the detected radiation into the appropriate energy subrange. Data for each subrange is stored in a plurality of transmission data memories. Reconstruction processors generate a transmission image representation representative of each energy subrange. A combine processor weights each energy subrange image representation with an assigned weighting factor to provide enhancement of at least one feature when the images are combined to generate weighted image representations. The plurality of transmission images are also combined with equal weighting to generate an image representation used to generate attenuation correction factors for correcting the emission data. A reconstruction processor generates a corrected emission image representation. The emission image can be combined with one of the feature-enhanced structural images using a combiner and displayed, allowing the functional emission image to be located with respect to structural or anatomical features. Also, a feature-enhanced structural image, can be used to register the emission image with an image from another modality, such as a computed tomography (CT) image.
U.S. Pat. No. 5,750,991 to Moyers et al. and U.S. Pat. No. 6,040,580 to Watson et al. disclose a method and apparatus for producing radioactive transmission measurements to form multi-dimensional attenuation correction data with a point source of radiation, such as required in positron emission tomography applications. This involves the passing of the point source proximate the face of a selected each of the tomograph units for the formation of a 3-D image, or a selected portion of the tomograph units for a 2-D image. As such, attenuation data, transmission data, detector performance data, etc., can be obtained. This point source of radiation, in one embodiment, is rapidly circulated through a conduit that passes across each detector face under the influence of a transport fluid in, for example, an oscillatory motion to achieve a selected radiation field whereby calculation of transmission measurements within a body positioned within the tomograph scanner is achieved. When not being circulated, the radiation source is held within a shield. Circulation of the transport fluid, typically a hydraulic fluid, is typically accomplished using a positive displacement pump. Position sensors are used to monitor the movement of the source in the conduit as well as its position within the shield. Disconnect units permit removal of the radiation source, as contained in the shield, from the system without accessing any other portions of the system.
Although the above references discloses various imaging techniques, improvements in hybrid medical imaging systems are desired. It is therefore an object of the present invention at least to provide a novel system and method for attenuation correction in hybrid medical imaging.